If ABC is a triangle, prove that the sum of its angles is 180°. [Hint: In the figure below...

If ABC is a triangle, prove that the sum of its angles is 180°. [Hint: In the figure below, use congruent triangles to show that ∠u ≅ ∠x and ∠v ≅ ∠y. You may assume that opposite sides of a rectangle have the same length.]